An intense sea operation to find the remains of an Air France jet that plunged into the Atlantic this week was being bolstered after days of fruitless searching, officials have said.

An intense sea operation to find the remains of an Air France jet that plunged into the Atlantic this week was being bolstered after days of fruitless searching, officials have said.

A French nuclear submarine was on its way to the zone, 1,000 km off Brazil's northeast coast, to help look for the black boxes from flight AF 447 which was lost on Monday as it flew from Rio de Janeiro to Paris with 228 people on board.

Two more Brazilian navy vessels late on Friday were also to join three others already in the area, which was being overflown by 12 Brazilian and French aircraft.

The head of air traffic control for the area, Brazilian Brigadier Ramon Cardoso, told reporters "we have not made any recovery of material."

Some items spotted floating in the vicinity were 'not relevant,' he said, adding that weather conditions were terrible, limiting visibility, and currents had changed direction.

Brazilian officials said items picked up on Thursday turned out on closer inspection to be nothing more than trash, probably from ships.

But positive sightings in the waves of a seat from a plane and cables and other components on Tuesday and Wednesday convinced searchers they were in the right spot.

Cardoso said those objects might have since sunk to the bottom of the ocean, where the plane's black boxes are also believed to be.